Atypical myopathy (also known as seasonal pasture myopathy and sycamore myopathy or poisoning) is an often fatal illness, usually found in grazing horses — mostly in the autumn and spring. This has been more widely recognised in recent years as more information about the disease has become available.
The condition weakens the muscles of the body, including the heart, and can present with sudden stiffness, muscle tremors, collapse and colic-like signs, accompanied by a low temperature. Often dark urine is seen because the damaged muscle cells release the pigment myoglobin, which can damage the kidneys.
Atypical myopathy has a fatality rate of around 70% so should always be considered a veterinary emergency if it is suspected.
Atypical myopathy is not infectious and can affect horses of all ages and types, although young horses may be more vulnerable. Some horses appear to be more susceptible than others, which may be due to genetic differences.
What causes atypical myopathy?
Several studies published during the past five to 10 years have established that atypical myopathy is caused by ingestion of the hypoglycin A toxin from the seeds, seedlings and possibly flowers of certain trees of the genus Acer. The most common of this type is the sycamore tree, but different trees contain variable amounts of the toxin. It is important to note that the toxin is not present in every seed, seedling or tree. The Royal Veterinary College provides a hypoglcyin A test which can be used on both plant samples and equine serum to identify the level of the toxin.